1. Field of the Invention
The present invention generally relates to resistance temperature detectors.
2. State of the Art
Resistance temperature detectors, or thermometers, are devices that sense temperature by means of changes in the magnitude of current through, or voltage across, an element whose electrical resistance varies with temperature. In many conventional resistance temperature detectors, the temperature-variable resistance element is fabricated from fine diameter wire (i.e., wires with diameters less than 50 um). Because of this construction, conventional resistance temperature detectors require substantial mechanical protection from chemical and/or physical hazards in their operating environments. For example, an operating environment might range in temperature from cryogenic temperatures to temperatures exceeding 400.degree. Centigrade.
Resistance temperature detectors are usually fabricated by winding a fine diameter wire on a bobbin or mandrel which, typically, is made of a ceramic material such as aluminum oxide (alumina). Then, the wire-wound bobbin is coated with an cement-like insulating material and is installed in a protective tube. In industrial applications, the protective tube is usually made of steel or a nickel alloy such as Monel. In some instances, the wire-wound mandrel is mounted in a ceramic tube for protection.
The above-described techniques for protecting resistance temperature detectors have the shortcoming that the temperature sensor in the temperature detector--the fine diameter resistance wire--is not only mechanically insulated but is also, to some extent, thermally removed from its operating environment by a relatively large mass. Generally speaking, the greater the mass of material separating the temperature sensor from its environment, or supporting the sensor in its environment, the slower is the response of the sensor to temperature changes. (Conventional resistance temperature detectors have a time constant of about 5 to 10 seconds in response to a step change in temperature in water flowing at 1 m/s.)
In practice, resistance temperature detectors have been constructed in various ways to reduce their thermal response times. For example, bobbin walls in resistance temperature detectors have been made very thin to minimize thermal mass and, accordingly, to decrease the time required for the temperature detector to reach equilibrium. As another example, protective housings have been made of materials that have very high thermal conductivity, such as silver alloys. Nevertheless, conventional resistance temperature detectors have thermal response times (i.e., time constants) that are too long for use in various applications and control systems, especially ones where response time is an issue because of safety concerns.